Hey Darrell - I live in Maine, how is it across the bay?
Anyways, I used powdered laundry detergent (plain TIDE) with a splash of bleach. Worked great with a RubberMaid brand deck brush. The key is to use a medium bristle brush, car washing brushes won't take the abuse and won't clean deep into the rubber.
This worked out well for me. I also have a yellow stain, from the AC unit down the roof. The TIDE/bleach combo took it out. I also used this same mixture in a higher concentration on the slide awnings and awning.
Be sure to wash your trailer down after.